The molecular basis of insecticide resistance in mosquitoes

Bio-efficacy of aluminum phosphide and cypermethrin against some physiological and biochemical aspects of Chrysomya megacephala maggots

Carrion flies play a significant role in forensic entomotoxicology, where they are employed as alternative samples when traditional samples are unavailable. In situations of poisoned death, these toxins disrupt insect development and affect forensic entomology analyses. So, forensic entomotoxicologists must be aware of this impact. The present study aimed to determine the effects of aluminum phosphide (AlP) and cypermethrin (CP) on the biochemical parameters and antioxidant enzymes of the third instar of Chrysomya megacephala maggots. C. megacephala was reared on normal and poisoned rabbit carcasses with aluminum phosphide and cypermethrin. The third larval instar of C. megacephala was studied using by spectrophotometer for detection of total protein, (TP), aspartate aminotransferase (AST), alanine aminotransferase (ALT), total antioxidant capacity (TAC), superoxide dismutase (SOD), glutathione s-transferase (GST), catalase (CAT) and malondialdehyde (MDA). The results indicated to significantly decrease of TP, TAC, SOD, GST and CAT and increase of AST, ALT and MDA in the maggots reared on the poisoned carcasses with AlP or CP compared with control group. In conclusion, the tested insecticides brought about a decrease antioxidant enzyme activity and increase of MDA could be involved in free radicals in C. megacephala larvae leading to oxidative stress by these insecticidal components.

Bacillus thuringiensis Cyt Proteins as Enablers of Activity of Cry and Tpp Toxins against Aedes albopictus

Aedes albopictus is a species of mosquito, originally from Southeast Asia, that belongs to the Culicidae family and the Dipteran insect order. The distribution of this vector has rapidly changed over the past decade, making most of the temperate territories in the world vulnerable to important human vector-borne diseases such as dengue, yellow fever, zika or chikungunya. Bacillus thuringiensis var. israeliensis (Bti)-based insecticides represent a realistic alternative to the most common synthetic insecticides for the control of mosquito larvae. However, several studies have revealed emerging resistances to the major Bti Crystal proteins such as Cry4Aa, Cry4Ba and Cry11Aa, making the finding of new toxins necessary to diminish the exposure to the same toxicity factors overtime. Here, we characterized the individual activity of Cyt1Aa, Cry4Aa, Cry4Ba and Cry11Aa against A. albopictus and found a new protein, Cyt1A-like, that increases the activity of Cry11Aa more than 20-fold. Additionally, we demonstrated that Cyt1A-like facilitates the activity three new Bti toxins: Cry53-like, Cry56A-like and Tpp36-like. All in all, these results provide alternatives to the currently available Bti products for the control of mosquito populations and position Cyt proteins as enablers of activity for otherwise non-active crystal proteins.

Effects of azadirachtin on detoxification-related gene expression in the fat bodies of the fall armyworm, Spodoptera frugiperda

The fall armyworm, Spodoptera frugiperda, has become a worldwide pest and threatens world food production. A previous study indicated that azadirachtin, the most effective botanical insecticide for S. frugiperda, inhibits larval growth of the insect. The effect of azadirachtin on the tissues of the larvae, however, remains to be determined. In this study, the effects of azadirachtin on the structure of fat bodies were analyzed. Comparative transcriptomic analysis was conducted between controls and samples treated with 0.1 μg/g azadirachtin for 7 days to explore potential relevant mechanisms. The expression of 5356 genes was significantly affected after azadirachtin treatment, with 3020 up-regulated and 2336 down-regulated. Among them, 137 encode detoxification enzymes, including 53 P450s, 20 GSTs, 27 CarEs, 16 UGTs, and 12 ABC transporters. Our results indicated that azadirachtin could destroy fat body structure and change the mRNA levels of detoxification-related genes. The up-regulated genes encoding detoxification enzymes might be related to detoxifying azadirachtin. Our results elucidate a preliminary mechanism of azadirachtin detoxification in the fat bodies of S. frugiperda larvae.

Predicting the insecticide-driven mutations in a crop pest insect: Evidence for multiple polymorphisms of acetylcholinesterase gene with potential relevance for resistance to chemicals

The silverleaf whitefly Bemisia tabaci (Gennadius, 1889) (Homoptera: Aleyrodidae) is a serious invasive herbivorous insect pest worldwide. The excessive use of pesticides has progressively selected B. tabaci specimens, reducing the effectiveness of the treatments, and ultimately ending in the selection of pesticide-resistant strains. The management of this crop pest has thus become challenging owing to the level of resistance to all major classes of recommended insecticides. Here, we used in silico techniques for detecting sequence polymorphisms in ace1 gene from naturally occurring B. tabaci variants, and monitor the presence and frequency of the detected putative mutations from 30 populations of the silverleaf whitefly from Egypt and Pakistan. We found several point mutations in ace1-type acetylcholinesterase (ace1) in the studied B. tabaci variants naturally occurring in the field. By comparing ace1 sequence data from an organophosphate-susceptible and an organophosphate-resistant strains of B. tabaci to ace1 sequence data retrieved from GenBank for that species and to nucleotide polymorphisms from other arthropods, we identified novel mutations that could potentially influence insecticide resistance. Homology modeling and molecular docking analyses were performed to determine if the mutation-induced changes in form 1 acetylcholinesterase (AChE1) structure could confer resistance to carbamate and organophosphate insecticides. Mutations had small effects on binding energy (ΔG_b) interactions between mutant AChE1 and insecticides; they altered the conformation of the peripheral anionic site of AChE1, and modified the enzyme surface, and these changes have potential effects on the target-site sensitivity. Altogether, the results from this study provide information on genic variants of B. tabaci ace1 for future monitoring insecticide resistance development and report a potential case of environmentally driven gene variations.

Multi-country evaluation of the durability of pyrethroid plus piperonyl-butoxide insecticide-treated nets: study protocol

BACKGROUND

Mass distributions of long-lasting insecticidal nets (LLINs) have contributed to large reductions in the malaria burden. However, this success is in jeopardy due in part to the increasing pyrethroid-resistant mosquito population as well as low LLINs coverage in various areas because the lifespan of LLINs is often shorter than the interval between replenishment campaigns. New insecticide-treated nets (ITNs) containing pyrethroid and piperonyl-butoxide (PBO) have shown a greater reduction in the incidence of malaria than pyrethroid LLINs in areas with pyrethroid-resistant mosquitoes. However, the durability (attrition, bio-efficacy, physical integrity and chemical retainment) of pyrethroid-PBO ITNs under operational settings has not been fully characterized. This study will measure the durability of pyrethroid-PBO ITNs to assess whether they meet the World Health Organization (WHO) three years of operational performance criteria required to be categorized as "long-lasting".

METHODS

A prospective household randomized controlled trial will be conducted simultaneously in Tanzania, India and Côte d'Ivoire to estimate the field durability of three pyrethroid-PBO ITNs (Veeralin®, Tsara® Boost, and Olyset® Plus) compared to a pyrethroid LLIN: MAGNet®. Durability monitoring will be conducted up to 36 months post-distribution and median survival in months will be calculated. The proportion of ITNs: (1) lost (attrition), (2) physical integrity, (3) resistance to damage score, (4) meeting WHO bio-efficacy (≥ 95% knockdown after 1 h or ≥ 80% mortality after 24 h for WHO cone bioassay, or ≥ 90% blood-feeding inhibition or ≥ 80% mortality after 24 h for WHO Tunnel tests) criteria against laboratory-reared resistant and susceptible mosquitoes, and insecticidal persistence over time will be estimated. The non-inferiority of Veeralin® and Tsara® Boost to the first-in-class, Olyset® Plus will additionally be assessed for mortality, and the equivalence of 20 times washed ITNs compared to field aged ITNs will be assessed for mortality and blood-feeding inhibition endpoints in the Ifakara Ambient Chamber Test, Tanzania.

CONCLUSION

This will be the first large-scale prospective household randomized controlled trial of pyrethroid-PBO ITNs in three different countries in East Africa, West Africa and South Asia, simultaneously. The study will generate information on the replenishment intervals for PBO nets.

Impact of insecticide resistance on malaria vector competence: a literature review

Since its first report in Anopheles mosquitoes in 1950s, insecticide resistance has spread very fast to most sub-Saharan African malaria-endemic countries, where it is predicted to seriously jeopardize the success of vector control efforts, leading to rebound of disease cases. Supported mainly by four mechanisms (metabolic resistance, target site resistance, cuticular resistance, and behavioural resistance), this phenomenon is associated with intrinsic changes in the resistant insect vectors that could influence development of invading Plasmodium parasites. A literature review was undertaken using Pubmed database to collect articles evaluating directly or indiretly the impact of insecticide resistance and the associated mechanisms on key determinants of malaria vector competence including sialome composition, anti-Plasmodium immunity, intestinal commensal microbiota, and mosquito longevity. Globally, the evidence gathered is contradictory even though the insecticide resistant vectors seem to be more permissive to Plasmodium infections. The actual body of knowledge on key factors to vectorial competence, such as the immunity and microbiota communities of the insecticide resistant vector is still very insufficient to definitively infer on the epidemiological importance of these vectors against the susceptible counterparts. More studies are needed to fill important knowledge gaps that could help predicting malaria epidemiology in a context where the selection and spread of insecticide resistant vectors is ongoing.

Cuticular profiling of insecticide resistant Aedes aegypti

Insecticides have made great strides in reducing the global burden of vector-borne disease. Nonetheless, serious public health concerns remain because insecticide-resistant vector populations continue to spread globally. To circumvent insecticide resistance, it is essential to understand all contributing mechanisms. Contact-based insecticides are absorbed through the insect cuticle, which is comprised mainly of chitin polysaccharides, cuticular proteins, hydrocarbons, and phenolic biopolymers sclerotin and melanin. Cuticle interface alterations can slow or prevent insecticide penetration in a phenomenon referred to as cuticular resistance. Cuticular resistance characterization of the yellow fever mosquito, Aedes aegypti , is lacking. In the current study, we utilized solid-state Nuclear Magnetic Resonance (ssNMR) spectroscopy, gas chromatography/mass spectrometry (GC-MS), and transmission electron microscopy (TEM) to gain insights into the cuticle composition of congenic cytochrome P450 monooxygenase insecticide resistant and susceptible Ae. aegypti . No differences in cuticular hydrocarbon content or phenolic biopolymer deposition were found. In contrast, we observed cuticle thickness of insecticide resistant Ae. aegypti increased over time and exhibited higher polysaccharide abundance. Moreover, we found these local cuticular changes correlated with global metabolic differences in the whole mosquito, suggesting the existence of novel cuticular resistance mechanisms in this major disease vector.

Hydroxyapatite-binding Silver/Titanium Dioxide as a Potential Control Compound Against Mosquito Vectors, Aedes aegypti (Diptera: Culicidae) and Anopheles dirus (Diptera: Culicidae)

Controlling mosquitoes is vital for counteracting the rising number of mosquito-borne illnesses. Vector control requires the implementation of various measures; however, current methods lack complete effectiveness, and new control agents or substances are urgently needed. Therefore, this study developed a nonwoven fabric sheet coated with hydroxyapatite-binding silver/titanium dioxide compound (hydroxyapatite-binding silver/titanium dioxide sheet [HATS])and evaluated its effectiveness on all stages of laboratory Aedes aegypti (Linnaeus); Diptera: Culicidae and Anopheles dirus (Peyton & Harrison); Diptera: Culicidae. We reared larvae with HATS and control sheets and assessed their mortality, emergence, and hatching rates. The submersion rates of engorged female mosquitoes in submerged HATS and control sheets were also compared. The HATS strongly affected mosquito development, resulting in high mortality rates (mean ± SE) of 99.66 ± 0.58% (L1-L2) and 91.11 ± 9.20% (L3-L4) for Ae. aegypti and 100% of both stages for An. dirus. In contrast, mosquitoes raised in the control sheet showed relatively high survival rates of 92.33 ± 3.21% (L1-L2) and 95.67 ± 0.58% (L3-L4) for Ae. aegypti and 86.07 ± 3.53% (L1-L2) and 92.01 ± 8.67% (L3-L4) for An. dirus. Submersion of engorged females was found in the HATS oviposition cup, leading to a decreased number of eggs and a low hatching rate compared to that of the control. Overall, HATS may be a useful new control method for Ae. aegypti and An. dirus.

Genetic Diversity of Cytochrome P450s CYP6M2 and CYP6P4 Associated with Pyrethroid Resistance in the Major Malaria Vectors Anopheles coluzzii and Anopheles gambiae from Yaoundé, Cameroon

Assessing the genetic diversity of metabolic resistance genes, such as cytochrome P450s, helps to understand the dynamics and evolution of resistance in the field. Here, we analyzed the polymorphisms of CYP6M2 and CYP6P4, associated with pyrethroid resistance in Anopheles coluzzii and Anopheles gambiae, to detect potential resistance markers. Field-caught resistant mosquitos and susceptible lab strains were crossed, and F4 was exposed to permethrin for 15 min and 90 min to discriminate highly susceptible (HS) and highly resistant (HR) mosquitos, respectively. Significant permethrin mortality reduction was observed after pre-exposure to PBO, suggesting the gene involvement of P450s. qPCR analysis revealed significant overexpression of CYP6M2 (FC = 19.57 [95% CI 13.96-25.18] for An. coluzzii; 10.16 [7.86-12.46] for An. gambiae) and CYP6P4 (FC = 6.73 [6.15-7.30] An. coluzzii; 23.62 [26.48-20.76] An. gambiae). Full-gene and ≈1 kb upstream were sequenced. For CYP6M2, the upstream region shows low diversity in HR and HS (overall Hd = 0.49, π = 0.018), whereas the full-gene shows allelic-variation but without evidence of ongoing selection. CYP6P4 upstream region showed a lower diversity in HR (Hd = 0.48) than HS (Hd = 0.86) of An. gambiae. These results highlighted that CYP6P4-associated resistance is potentially driven by modification in upstream region. However, further work is needed to determine the real causative variants that will help design rapid detection tools.

Spatial distribution and insecticide resistance profile of Aedes aegypti and Aedes albopictus in Douala, the most important city of Cameroon

Prevention and control of Aedes-borne viral diseases such as dengue rely on vector control, including the use of insecticides and reduction of larval sources. However, this is threatened by the emergence of insecticide resistance. This study aimed to update the spatial distribution, the insecticide resistance profile of A. aegypti and A. albopictus and the potential resistant mechanisms implicated in the city of Douala. Immature stages of Aedes were collected in August 2020 in eight neighbourhoods in Douala and reared to adult stages. Adult bioassays, and piperonyl butoxide (PBO) synergist assays were carried out according to World Health Organization recommendations. Expression of some candidate metabolic genes including Cyp9M6F88/87, Cyp9J28a, Cyp9J10 and Cyp9J32 in A. aegypti, and Cyp6P12 in A. albopictus were assessed using qPCR. A. aegypti adults G0 were screened using real time melting curve qPCR analyses to genotype the F1534C, V1016I and V410L Aedes kdr mutations. Overall, A. aegypti is the predominant Aedes species, but analyses revealed that both A. albopictus and A. aegypti coexist in all the prospected neighbourhoods of Douala. High level of resistance was observed to three pyrethroids tested in both Aedes species. In A. aegypti a lower mortality rate was reported to permethrin (5.83%) and a higher mortality rate to deltamethrin (63.74%). Meanwhile, for A. albopictus, lower (6.72%) and higher (84.11%) mortality rates were reported to deltamethrin. Similar analysis with bendiocarb, revealed for A. aegypti a loss of susceptibility. However, in A. albopictus samples, analyses revealed a susceptibility in Logbessou, and confirmed resistance in Kotto (59.78%). A partial recovery of mortality was found to insecticides after pre-exposure to PBO. Cyp6P12 was found significantly overexpressed in A. albopictus permethrin resistant and Cyp9M6F88/87 for A. aegypti deltamethrin resistant. F1534C, V1016I and V410L mutations were detected in A. aegypti from different neighbourhoods and by considering the combination of these three kdr 14 genotypes were found. These findings provide relevant information which should be capitalised in the implementation of arbovirus vector control strategies and insecticide resistance management.

Natural plant diet impacts phenotypic expression of pyrethroid resistance in Anopheles mosquitoes

Success in reducing malaria transmission through vector control is threatened by insecticide resistance in mosquitoes. Although the proximal molecular mechanisms and genetic determinants involved are well documented, little is known about the influence of the environment on mosquito resistance to insecticides. The aim of this study was to assess the effect of plant sugar feeding on the response of Anopheles gambiae sensu lato to insecticides. Adults were fed with one of four treatments, namely a 5% glucose control solution, nectariferous flowers of Barleria lupulina, of Cascabela thevetia and a combination of both B. lupulina + C. thevetia. WHO tube tests were performed with 0.05% and 0.5% deltamethrin, and knockdown rate (KD) and the 24 h mosquito mortality were measured. Plant diet significantly influenced mosquito KD rate at both concentrations of deltamethrin. Following exposure to 0.05% deltamethrin, the B. lupulina diet induced a 2.5 fold-increase in mosquito mortality compared to 5% glucose. Species molecular identification confirmed the predominance of An. gambiae (60% of the samples) over An. coluzzii and An. arabiensis in our study area. The kdr mutation L1014F displayed an allelic frequency of 0.75 and was positively associated with increased phenotypic resistance to deltamethrin. Plant diet, particularly B. lupulina, increased the susceptibility of mosquitoes to insecticides. The finding that B. lupulina-fed control individuals (i.e. not exposed to deltamethrin) also displayed increased 24 h mortality suggests that plant-mediated effects may be driven by a direct effect of plant diet on mosquito survival rather than indirect effects through interference with insecticide-resistance mechanisms. Thus, some plant species may weaken mosquitoes, making them less vigorous and more vulnerable to the insecticide. There is a need for further investigation, using a wider range of plant species and insecticides, in combination with other relevant environmental factors, to better understand the expression and evolution of insecticide resistance.

Varroa destructor resistance to tau-fluvalinate: relationship between in vitro phenotypic test and VGSC L925V mutation

BACKGROUND

Varroa destructor is a parasitic mite of the honey bee, Apis mellifera. Its presence in colonies can lead to a collapse within a few years. The use of acaricides has become essential to manage the hive infestation. However, the repeated and possibly incorrect use of acaricide treatments, as tau-fluvalinate, has led to the development of resistance. The in vitro phenotypic test allows the proportion of susceptible or resistant individuals to be known following an exposure to an active substance. In Varroa mites, resistance to tau-fluvalinate is associated with the presence of mutations at the position 925 of the voltage-gated sodium channel (VGSC).

RESULTS

Here, we compared the results obtained with an in vitro phenotypic test against tau-fluvalinate and those obtained with an allelic discrimination assay on 13 treated and untreated Varroa populations in France. The correlation between the phenotype and the genetic profile rate is found to be 0.89 Varroa mites having resistant phenotypic profile have a probability of 63% to present the L925V mutation (resistance detection reliability). However, 97% of the Varroa mites having the susceptible phenotype do not present the L925V mutation (susceptible detection reliability).

CONCLUSION

The L925V mutation explains most of the resistance to tau-fluvalinate in V. destructor in the populations tested. However, other mutations or types of resistance may also be involved to explain the survival of Varroa mites in the phenotypic test. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Discovery of malathion resistance QTL in Drosophila melanogaster using a bulked phenotyping approach

Drosophila melanogaster has proved an effective system with which to understand the evolutionary genetics and molecular mechanisms of insecticide resistance. Insecticide use has left signatures of selection in the fly genome, and both functional and quantitative genetic studies in the system have identified genes and variants associated with resistance. Here, we use D. melanogaster and leverage a bulk phenotyping and pooled sequencing "extreme quantitative trait loci" approach to genetically dissect variation in resistance to malathion, an organophosphate insecticide. We resolve 2 quantitative trait loci, one of which implicates allelic variation at the cytochrome P450 gene Cyp6g1, a strong candidate based on previous work. The second shows no overlap with hits from a previous genome-wide association study for malathion resistance, recapitulating other studies showing that different strategies for complex trait dissection in flies can yield apparently different architectures. Notably, we see no genetic signal at the Ace gene. Ace encodes the target of organophosphate insecticide inhibition, and genome-wide association studies have identified strong Ace-linked associations with resistance in flies. The absence of quantitative trait locus implicating Ace here is most likely because our mapping population does not segregate for several of the known functional polymorphisms impacting resistance at Ace, perhaps because our population is derived from flies collected prior to the widespread use of organophosphate insecticides. Our fundamental approach can be an efficient, powerful strategy to dissect genetic variation in resistance traits. Nonetheless, studies seeking to interrogate contemporary insecticide resistance variation may benefit from deriving mapping populations from more recently collected strains.

Interactions between vector competence to chikungunya virus and resistance to deltamethrin in Aedes aegypti laboratory lines?

The urban mosquito species Aedes aegypti is the main vector of arboviruses worldwide. Mosquito control with insecticides is the most prevalent method for preventing transmission in the absence of effective vaccines and available treatments; however, the extensive use of insecticides has led to the development of resistance in mosquito populations throughout the world, and the number of epidemics caused by arboviruses has increased. Three mosquito lines with different resistance profiles to deltamethrin were isolated in French Guiana, including one with the I1016 knock-down resistant allele. Significant differences were observed in the cumulative proportion of mosquitoes with a disseminated chikungunya virus infection over time across these lines. In addition, some genes related to resistance (CYP6BB2, CYP6N12, GST2, trypsin) were variably overexpressed in the midgut at 7 days after an infectious bloodmeal in these three lines. Our work shows that vector competence for chikungunya virus varied between Ae. aegypti laboratory lines with different deltamethrin resistance profiles. More accurate verification of the functional association between insecticide resistance and vector competence remains to be demonstrated.

First Report of Mutations Associated With Pyrethroid (L1014F) and Organophosphate (G119S) Resistance in Belgian Culex (Diptera: Culicidae) Mosquitoes

The emergence of West Nile virus and Usutu virus in Europe poses a significant risk to public health. In the absence of efficient antiviral therapy or vaccine candidates, the only strategy to control these arboviruses is to target the Culex (Diptera: Culicidae) mosquito vector. However, the selection pressure caused by exposure to insecticides for vector control or agricultural pest control can lead to insecticide resistance, thereby reducing the efficacy of insecticide-based vector control interventions. In Culex mosquitoes, two of the most common amino acid substitutions associated with insecticide resistance are the kdr L1014F in voltage gated sodium channels and G119S in acetylcholinesterase. In this study, Culex pipiens biotype pipiens, Culex torrentium, and Culex modestus were sampled from 2019 to 2021 in three distinct environmental habitats (urban, peri-urban, and agricultural) in and around the city of Leuven, Belgium. Individual mosquitoes were screened for two mutations resulting in L1014F and G119S amino acid substitutions. Both mutations were observed in Cx. pipiens and Cx. modestus but not in Cx. torrentium mosquitoes across the four collection sites. Furthermore, multi-resistance or cross-resistance in Cx. pipiens could be a threat in these areas, as both mutations were observed at low frequencies. These results provide the first report of kdr L1014F and ace-1 G119S resistance mutations in Cx. pipiens and Cx. modestus mosquitoes from Belgium, highlighting the importance of mosquito surveillance to design effective arbovirus outbreak control strategies.

No association between habitat, autogeny and genetics in Moroccan Culex pipiens populations

Background

Mosquitoes of the Culex pipiens complex are found across the globe and are the focus of many research studies. Among the temperate species C. pipiens sensu stricto (s.s.), two forms are usually described: molestus and pipiens. These two forms are indistinguishable in terms of morphology but show behavioral and physiological differences that may have consequences for their associated epidemiology. The two forms are well defined in the northern part of the species distribution, where autogeny is strictly associated with the molestus form. However, whether the two remain distinct and show the characteristic differences in behavior is less clear in North Africa, at the southern edge of their range.

Methods

The association between autogeny, as determined by ovarian dissection, and molecular forms, based on the CQ11 microsatellite marker, was studied in six Moroccan populations of C. pipiens.

Results

An overall low prevalence of autogeny was found at three of the Moroccan regions studied, although it reached 17.5% in the Agadir population. The prevalence of form-specific CQ11 alleles was quite similar across all populations, with the molestus allele being rarer (approx. 15%), except in the Agadir population where it reached 43.3%. We found significant deficits in heterozygotes at the diagnostic CQ11 locus in three populations, but the three other populations showed no significant departure from panmixia, which is in line with the results of a retrospective analysis of the published data. More importantly, we found no association between the autogeny status and CQ11 genotypes, despite the many females analyzed.

Conclusions

There was limited evidence for two discrete forms in Morocco, where individuals carrying pipiens and molestus alleles breed and mate in the same sites and are equally likely to be capable of autogeny. These observations are discussed in the epidemiological context of Morocco, where C. pipiens is the main vector of several arboviruses.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05469-3.

Detection of L1014F knockdown resistance mutation in Culex tritaeniorhynchus populations

Culex tritaeniorhynchus is a major Japanese encephalitis virus vector distributed in Southeast Asia and surrounding countries. The aim of the present study is to investigate insecticide resistance status among 10 Cx. tritaeniorhynchus populations of the Mediterranean region of Turkey. Bioassay results indicated that all of the populations were resistant or at least possibly resistant to 1,1'-(2,2,2-Trichloroethane-1,1-diyl) bis (4-chlorobenzene) (DDT) (4%), [(dimethoxyphosphorothioyl) sulfanyl] butanedioate, Diethyl (malathion) (5%), and 2-[(Propan-2-yl) oxy] phenyl methylcarbamate (propoxur) (0,1%). Whereas, some of the populations were still susceptible to 3-Phenoxybenzyl (1RS)-cis, trans-3-(2,2-dichlorovinyl)-2,2-dimethyl cyclopropane carboxylate (permethrin) (0,75%) and (S)-Cyano (3-phenoxy phenyl) methyl (1R,3R)-3-(2,2-dibromoethen-1-yl)-2,2-dimethylcyclopropane-1-carboxylate (deltamethrin) (0,05%). Biochemical analysis results showed altered alpha esterase, beta esterase, para-nitrophenyl acetate (PNPA), and glutathione-s-transferase (GST) levels in some populations while all of the populations had increased oxidase levels except for the Yumurtalık population. Additionally, all of the populations had sensitive acetylcholinesterase (AChE) levels similar to the control group except for the Erzin population. Correlation analysis showed a significant correlation between mortality rates for deltamethrin and alpha esterase, beta esterase, PNPA, and GST levels while mortality rates for permethrin were significantly correlated with GST levels. An allele-specific polymerase chain reaction (AS-PCR) detected high L1014F allele frequency in the populations. Overall results indicate the urgent need for monitoring and mapping of insecticide resistance in Cx. tritaeniorhynchus populations of the study area for effective vector control management.

Fitness costs associated with a GABA receptor mutation conferring dieldrin resistance in Aedes albopictus

Understanding the dynamics of insecticide resistance genes in mosquito populations is pivotal for a sustainable use of insecticides. Dieldrin resistance in Aedes albopictus is conferred by the alanine to serine substitution (A302S or RdlR allele) in the γ-aminobutyric acid (GABA) receptor encoded by the Rdl gene. On Reunion Island, dieldrin resistance was initially reported in natural Ae. albopictus populations sampled in 2008 despite the ban of dieldrin since 1994. To monitor insecticide resistance in Ae. albopictus on the island and to identify its drivers, we measured (i) the frequency of resistance alleles in 19 distinct natural populations collected between 2016 and 2017, (ii) fitness costs associated with dieldrin resistance in laboratory-controlled experiments, and (iii) the resistance conferred by RdlR to fipronil, an insecticide widely used on the island and reported to cross-react with RdlR. The results show a persistence of RdlR in Ae. albopictus natural populations at low frequencies. Among the measured life history traits, mortality in pre-imaginal stages, adults' survival as well as the proportion of egg-laying females were significantly affected in resistant mosquitoes. Finally, bioassays revealed resistance of RdlR mosquitoes to fipronil, suggesting that the use of fipronil in natura could select for the RdlR allele. This study shows that dieldrin resistance is persistent in natural mosquito populations likely as a result of combined effects between fitness costs associated with RdlR and selection exerted by cross-reacting environmental insecticides such as fipronil.

Tandem duplication of a genomic region encoding glutathione S-transferase epsilon-2 and -4 genes in DDT-resistant Anopheles stephensi strain from India

The glutathione S-transferases (GST) genes are a multigene family of enzymes involved in the metabolism of endogenous and xenobiotic compounds by catalysing the conjugation of the reduced form of glutathione to the substrate. The epsilon class of GST (GSTe), unique to arthropods, is known to be involved in the detoxification process of several classes of insecticides, and GSTe2 in particular is known to have DDT dehydrochlorinase activity. This communication reports a tandem duplication of a genomic region encoding GSTe2 and GSTe4 genes in a laboratory-colonized DDT-resistant Anopheles stephensi. We identified duplication breakpoints and the organization of gene duplication through Sanger sequencing performed on long-PCR products. Manual annotation of sequences revealed a tandemly-arrayed duplication of a 3.62 kb segment of GST epsilon gene clusters comprised of five genes: a partial GSTe1, GSTe2, GSTe2-pseudogene, GSTe4 and partial GSTe5, interconnected by a conserved 2.42 kb DNA insert segment major part of which is homologous to a genomic region located on a different chromosome. The tandemly duplicated array contained a total of two GSTe2 and three GSTe4 functional paralog genes. Read-depth coverage and split-read analysis of Illumina-based whole-genome sequence reads confirmed the presence of duplication in the corresponding region of the genome. The increased gene dose in mosquitoes as a result of the GSTe gene-duplication may be an adaptive process to increase levels of detoxifying enzymes to counter insecticide pressure.

Updating the Insecticide Resistance Status of Aedes aegypti and Aedes albopictus in Asia: A Systematic Review and Meta-Analysis

Background: Aedes aegypti and Aedes albopictus are two important vectors of several important arboviruses, including the dengue, chikungunya, and Zika viruses. Insecticide application is an important approach to reduce vector abundance during Aedes spp.-borne outbreaks in the absence of effective vaccines and treatments. However, insecticide overuse can result in the development of resistance, and careful monitoring of resistance markers is required. Methods: This meta-analysis and systematic review explored the spatial and temporal patterns of insecticide resistance in Asia from 2000 to 2021. PubMed, Scopus, EbscoHost, and Embase were used to enhance the search capability. The random-effects model was applied for the 94 studies that met our inclusion criteria for qualitative synthesis and meta-analysis. Results: Four major insecticides were studied (malathion, dichlorodiphenyltrichloroethane, permethrin, and deltamethrin). Dichlorodiphenyltrichloroethane resistance rates were high in both Ae. aegypti and Ae. albopictus (68% and 64%, respectively). Conversely, malathion resistance was less prevalent in Ae. aegypti (3%), and deltamethrin resistance was less common in Ae. albopictus (2%). Ae. aegypti displayed consistently high resistance rates (35%) throughout the study period, whereas the rate of insecticide resistance in Ae. albopictus increased from 5% to 12%. The rates of the major kdr mutations F1534C, V1016G, and S989P were 29%, 26%, and 22%, respectively. Conclusions: Insecticide resistance in both Ae. aegypti and Ae. albopictus is widespread in Asia, although the rates vary by country. Continuous monitoring of the resistance markers and modification of the control strategies will be important for preventing unexpected outbreaks. This systematic review and meta-analysis provided up-to-date information on insecticide resistance in dengue-endemic countries in Asia.

Rapid detection of kdr mutation F1534C in Aedes aegypti using recombinase polymerase amplification and lateral flow dipsticks

Insecticide resistance monitoring is essential in assessing the efficacy of vector control measures. However, gold standard PCR-based molecular analyses for insecticide resistance detection are often hindered by time-consuming sample processing, as well as considerable infrastructure and resourcing requirements. In this study, we combined a novel one-step sample preparation reagent with a rapid isothermal molecular test that detects a knock down resistance (kdr) mutation (F1534C) that enables pyrethroid resistance in Aedes aegypti mosquitoes. We trialled the rapid F1534C pyrethroid resistance test using insecticide resistant Ae. aegypti mosquito bodies and compared results to a conventional, allele-specific quantitative PCR (AS-qPCR) coupled with melt curve genotyping in corresponding mosquito heads. From a strain of Ae. aegypti established from an insecticide resistant population in Merida, Mexico (n = 27), all the mosquito bodies (n = 27) tested positive with the rapid F1534C test regardless of whether they were homozygous or heterozygous. To assess diagnostic test specificity, we confirmed that F1534 was not detected in laboratory-reared, fully susceptible Ae. aegypti mosquito bodies (n = 28) using the rapid F1534C test or the conventional AS-qPCR melt curve analysis. All corresponding mosquito heads (n = 28) were homozygous wild-type FF1534. The rapid F1534C test thus demonstrated 100% diagnostic sensitivity (95% CI: 87.23% to 100%) and 100% diagnostic specificity (95% CI: 87.66% to 100.00%) for detection of the F1534C pyrethroid resistant single nucleotide polymorphism (SNP) in both heterozygous and homozygous Ae. aegypti. In the collection of mutant mosquitoes from Mexico, CC1534 homozygous mutants occurred at a frequency of 74.1% (n = 20) and FC heterozygous mutants at a frequency of 25.9% (n = 7). The rapid F1534C test significantly reduced the sample processing and testing time from approximately 6 h for the AS-qPCR melt curve analysis to only 25 min. These results demonstrate significant potential for our approach to resistance testing as a field-based, low-resource, rapid alternative to time-consuming and expensive laboratory-based detection.

Detection of Voltage-Gated Sodium Channel (VGSC) Mutation in Deltamethrin-Resistant Blattella germanica L. from Indonesia

<b>Background and Objective:</b> German cockroach (<i>Blattella germanica</i> L.) is one of the most common residential pests in Indonesia. Controlling the population face obstacles due to insecticide resistance, especially to deltamethrin. This research investigated the resistance status and the possibility of a Voltage-Gated Sodium Channel (VGSC) knockdown resistance mutation (L1014F) in two field strains of German cockroaches collected from two cities in Indonesia (Bukittinggi, named RMKN-BKT and Bandung, named KRSA-BDG) with VCRU-WHO as the standard strain. <b>Materials and Methods:</b> This study started with a bioassay test to determine the lethal dose of 50% (LD<sub>50</sub>) for each strain and followed by a molecular test for mutation detection. <b>Results:</b> The results showed that the RMKN-BKT and KRSA-BDG strains were highly resistant to deltamethrin with RR50 values of 80,090 times and 73,272 times, respectively. Only the RMKN-BKT strain was shown to carry L1014F kdr mutation which lead to an amino acid replacement from leucine (TTG) to phenylalanine (TTC). Two silent mutations were also found in both field strains at codons 983 (TGC/cysteine→TGT/cysteine) and 984 (GGG/ glycine→GGA/glycine) which were suggested as polymorphism phenomena. The absence of the L1014F mutation in the Bandung strain does not exclude the possibility of the presence of the VGSC mutation at other points. <b>Conclusion:</b> It requires subsequent investigation in mutation detection at other points and the possible presence of other resistance mechanisms to get a precise solution in the population control. Bioinsecticides may stand as a breakthrough so that the strategy will no longer focus on insecticides.

Rapid Evaporative Ionization Mass Spectrometry (REIMS): a Potential and Rapid Tool for the Identification of Insecticide Resistance in Mosquito Larvae

Insecticide resistance is a significant challenge facing the successful control of mosquito vectors globally. Bioassays are currently the only method for phenotyping resistance. They require large numbers of mosquitoes for testing, the availability of a susceptible comparator strain, and often insectary facilities. This study aimed to trial the novel use of rapid evaporative ionization mass spectrometry (REIMS) for the identification of insecticide resistance in mosquitoes. No sample preparation is required for REIMS and analysis can be rapidly conducted within hours. Temephos resistant Aedes aegypti (Linnaeus) larvae from Cúcuta, Colombia and temephos susceptible larvae from two origins (Bello, Colombia, and the lab reference strain New Orleans) were analyzed using REIMS. We tested the ability of REIMS to differentiate three relevant variants: population source, lab versus field origin, and response to insecticide. The classification of these data was undertaken using linear discriminant analysis (LDA) and random forest. Classification models built using REIMS data were able to differentiate between Ae. aegypti larvae from different populations with 82% (±0.01) accuracy, between mosquitoes of field and lab origin with 89% (±0.01) accuracy and between susceptible and resistant larvae with 85% (±0.01) accuracy. LDA classifiers had higher efficiency than random forest with this data set. The high accuracy observed here identifies REIMS as a potential new tool for rapid identification of resistance in mosquitoes. We argue that REIMS and similar modern phenotyping alternatives should complement existing insecticide resistance management tools.

Fitness cost of malathion resistance in Egyptian Culex pipiens populations

The extensive use of insecticides in agriculture and public health has resulted in the rapid development of insecticide resistance in mosquito populations. Therefore, in this study, we aimed to evaluate insecticide resistance costs on the fitness of Culex pipiens. Two Cx. pipiens field populations (Beheira malathion-resistant and Gharbia malathion-susceptible) were compared to the reference (sensitive) population. The biochemical composition and expression of four genes relevant to insecticide resistance were estimated in third instar larvae. Adult survival, female fecundity and egg hatchability were also determined. As per our findings, it was found that the total protein and carbohydrate contents in Beheira malathion-resistant larvae were significantly lower than that in the reference larvae. Beheira malathion-resistant larvae had higher phenoloxidase (PO) specific activity than the reference population. In terms of the relevant genes, only cytochrome P450 (CYP6F1) expression showed elevated levels in the Gharbia malathion-susceptible population compared to the Beheira malathion-resistant population. In esterases (Estα and Estβ) and glutathione S-transferase, the tested populations did not show any significant differences. Compared to the reference mosquito population, Gharbia malathion-susceptible Cx. pipiens males exhibited significantly longer median survival. Female fecundity and hatchability showed nonsignificant differences among the populations tested. In conclusion, malathion resistance can induce lower protein and carbohydrate contents, but higher PO activity in larvae.

Spatial distribution of insecticide resistant populations of Aedes aegypti and Ae. albopictus and first detection of V410L mutation in Ae. aegypti from Cameroon

Background

Dengue (DENV), chikungunya (CHIKV) and Zika virus (ZIKV), are mosquito-borne viruses of medical importance in most tropical and subtropical regions. Vector control, primarily through insecticides, remains the primary method to prevent their transmission. Here, we evaluated insecticide resistance profiles and identified important underlying resistance mechanisms in populations of Aedes aegypti and Ae. albopictus from six different regions in Cameroon to pesticides commonly used during military and civilian public health vector control operations.

Methods

Aedes mosquitoes were sampled as larvae or pupae between August 2020 and July 2021 in six locations across Cameroon and reared until the next generation, G1. Ae. aegypti and Ae. albopictus adults from G1 were tested following World Health Organization (WHO) recommendations and Ae. aegypti G0 adults screened with real time melting curve qPCR analyses to genotype the F1534C, V1016I and V410L Aedes kdr mutations. Piperonyl butoxide (PBO) assays and real time qPCR were carried out from some cytochrome p450 genes known to be involved in metabolic resistance. Statistical analyses were performed using Chi-square test and generalized linear models.

Results

Loss of susceptibility was observed to all insecticides tested. Mortality rates from tests with 0.25% permethrin varied from 24.27 to 85.89% in Ae. aegypti and from 17.35% to 68.08% in Ae. albopictus. Mortality rates for 0.03% deltamethrin were between 23.30% and 88.20% in Ae. aegypti and between 69.47 and 84.11% in Ae. albopictus. We found a moderate level of resistance against bendiocarb, with mortality rates ranging from 69.31% to 90.26% in Ae. aegypti and from 86.75 to 98.95% in Ae. albopictus. With PBO pre-exposure, we found partial or fully restored susceptibility to pyrethroids and bendiocarb. The genes Cyp9M6F88/87 and Cyp9J10 were overexpressed in Ae. aegypti populations from Douala sites resistant to permethrin and deltamethrin. Cyp6P12 was highly expressed in alphacypermethrin and permethrin resistant Ae. albopictus samples. F1534C and V1016I mutations were detected in A. aegypti mosquitoes and for the first time V410L was reported in Cameroon.

Conclusions

This study revealed that Ae. aegypti and Ae. albopictus are resistant to multiple insecticide classes with multiple resistance mechanisms implicated. These findings could guide insecticide use to control arbovirus vectors in Cameroon.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40249-022-01013-8.

A statistical calibration tool for methods used to sample outdoor-biting mosquitoes

Background

Improved methods for sampling outdoor-biting mosquitoes are urgently needed to improve surveillance of vector-borne diseases. Such tools could potentially replace the human landing catch (HLC), which, despite being the most direct option for measuring human exposures, raises significant ethical and logistical concerns. Several alternatives are under development, but detailed evaluation still requires common frameworks for calibration relative to HLC. The aim of this study was to develop and validate a statistical framework for predicting human-biting rates from different exposure-free alternatives.

Methods

We obtained mosquito abundance data (Anopheles arabiensis, Anopheles funestus and Culex spp.) from a year-long Tanzanian study comparing six outdoor traps [Suna Trap (SUN), BG Sentinel (BGS), M-Trap (MTR), M-Trap + CDC (MTRC), Ifakara Tent Trap-C (ITT-C) and Mosquito Magnet-X Trap (MMX)] and HLC. Generalised linear models were developed within a Bayesian framework to investigate associations between the traps and HLC, taking intra- and inter-specific density dependence into account. The best model was used to create a calibration tool for predicting HLC-equivalents.

Results

For An. arabiensis, SUN catches had the strongest correlation with HLC (R² = 19.4), followed by BGS (R² = 17.2) and MTRC (R² = 13.1) catches. The least correlated catch was MMX (R² = 2.5). For An. funestus, BGS had the strongest correlation with the HLC (R² = 53.4), followed by MTRC (R² = 37.4) and MTR (R² = 37.4). For Culex mosquitoes, the traps most highly correlated with the HLC were MTR (R² = 45.4) and MTRC (R² = 44.2). Density dependence, both between and within species, influenced the performance of only BGS traps. An interactive Shiny App calibration tool was developed for this and similar applications.

Conclusion

We successfully developed a calibration tool to assess the performance of different traps for assessing outdoor-biting risk, and established a valuable framework for estimating human exposures based on the trap catches. The performance of candidate traps varied between mosquito taxa; thus, there was no single optimum. Although all the traps tested underestimated the HLC-derived exposures, it was possible to mathematically define their representativeness of the true biting risk, with or without density dependence. The results of this study emphasise the need to aim for a consistent and representative sampling approach, as opposed to simply seeking traps that catch the most mosquitoes.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05403-7.

Identification and expression patterns of candidate carboxylesterases in Carposina sasakii Matsumura (Lepidoptera: Carposinidae), an important pest of fruit trees

Carposina sasakii Matsumura (Lepidoptera: Carposinidae) is an important pest of fruit trees in a large area of Asia. The adults mainly depend on olfaction to communicate with the environment, but the olfactory mechanism has not been well known. Odorant degrading enzymes (ODEs) are important olfactory proteins, which inactivate and degrade odorants to free odorant receptors for maintaining olfactory sensitivity. Carboxylesterases (CXEs) are considered to be a major group of moth ODEs. In this study, four candidate CXEs (CsasCXE1 ~ CsasCXE4) were identified by using head transcriptomic data from C. sasakii adult females and males. Sequence alignment showed conserved amino acid residues and their variations in C. sasakii CXEs. Phylogenetic analysis indicated the CXEs with the variations cluster well, and each C. sasakii CXE clusters in a clade with some of the other lepidopteran CXEs, with a high enough bootstrap value. Gene expression analysis revealed that CsasCXE2 and CsasCXE3 have similar tissue and sex expression patterns in C. sasakii adults. The two CXEs have relatively high expression levels in the heads and are expressed more abundantly in the female heads than male heads. CsasCXE1 and CsasCXE4 are expressed at higher levels in the male heads than female heads, but not dominantly expressed in the heads among the different tissues. Whether these CXEs function as ODEs remains to be further researched. This study laid the foundation for exploring functions of C. sasakii CXEs.

Effects of agricultural pesticides on the susceptibility and fitness of malaria vectors in rural south-eastern Tanzania

BACKGROUND

Agricultural pesticides may exert strong selection pressures on malaria vectors during the aquatic life stages and may contribute to resistance in adult mosquitoes. This could reduce the performance of key vector control interventions such as indoor-residual spraying and insecticide-treated nets. The aim of this study was to investigate effects of agrochemicals on susceptibility and fitness of the malaria vectors across farming areas in Tanzania.

METHODS

An exploratory mixed-methods study was conducted to assess pesticide use in four villages (V1-V4) in south-eastern Tanzania. Anopheles gambiae (s.l.) larvae were collected from agricultural fields in the same villages and their emergent adults examined for insecticide susceptibility, egg-laying and wing lengths (as proxy for body size). These tests were repeated using two groups of laboratory-reared An. arabiensis, one of which was pre-exposed for 48 h to sub-lethal aquatic doses of agricultural pesticides found in the villages.

RESULTS

Farmers lacked awareness about the linkages between the public health and agriculture sectors but were interested in being more informed. Agrochemical usage was reported as extensive in V1, V2 and V3 but minimal in V4. Similarly, mosquitoes from V1 to V3 but not V4 were resistant to pyrethroids and either pirimiphos-methyl or bendiocarb, or both. Adding the synergist piperonyl butoxide restored potency of the pyrethroids. Pre-exposure of laboratory-reared mosquitoes to pesticides during aquatic stages did not affect insecticide susceptibility in emergent adults of the same filial generation. There was also no effect on fecundity, except after pre-exposure to organophosphates, which were associated with fewer eggs and smaller mosquitoes. Wild mosquitoes were smaller than laboratory-reared ones, but fecundity was similar.

CONCLUSIONS

Safeguarding the potential of insecticide-based interventions requires improved understanding of how agricultural pesticides influence important life cycle processes and transmission potential of mosquito vectors. In this study, susceptibility of mosquitoes to public health insecticides was lower in villages reporting frequent use of pesticides compared to villages with little or no pesticide use. Variations in the fitness parameters, fecundity and wing length marginally reflected the differences in exposure to agrochemicals and should be investigated further. Pesticide use may exert additional life cycle constraints on mosquito vectors, but this likely occurs after multi-generational exposures.

Physiological Responses of the Firefly Pyrocoelia analis (Coleoptera: Lampyridae) to an Environmental Residue From Chemical Pesticide Imidacloprid

Imidacloprid, a neonicotinoid insecticide, is widely applied to control insect pests across a broad spectrum. Though the impact of residues from this chemical pesticide on non-target organisms in the field has been reported, it was not well characterized across a wide range of ecosystems, especially for some species considered as environmental indicators that live in forests. The effects of sublethal dose of imidacloprid on firefly, Pyrocoelia analis, were analyzed physiologically and biochemically in this study to better understand the impact of chemical pesticide application on environmental indicators such as fireflies. After imidacloprid treatment, the midgut tissues of the larva presented an abnormal morphology featured as atrophy of fat body cells, shrinking cells, and the destruction of a midgut structure. The activities of antioxidant enzymes, superoxide dismutase, catalase, and peroxidase were noticeably increased during early exposure to sublethal imidacloprid and then decreased at later stages. The malondialdehyde content significantly increased after 12 h of exposure to imidacloprid compared with the control. Similarly, the enzyme activities of polyphenol oxidase and acetylcholinesterase were increased after the imidacloprid treatment and then decreased at the later stage. In summary, a sublethal dose of imidacloprid caused destructive change in the tissue structure, and this damage was followed by an excessive reactive oxygen species that could not be eliminated by antioxidant enzymes. Our results indicated that the residues of imidacloprid might cause severe toxicity to non-target insects in the environment even far away from the agro-ecosystem where the chemicals were applied.

Management of avian malaria in populations of high conservation concern

Avian malaria is a vector-borne disease that is caused by Plasmodium parasites. These parasites are transmitted via mosquito bites and can cause sickness or death in a wide variety of birds, including many threatened and endangered species. This Primer first provides contextual background for the avian malaria system including the life cycle, geographic distribution and spread. Then, we focus on recent advances in understanding avian malaria ecology, including how avian malaria can lead to large ecosystem changes and variation in host immune responses to Plasmodium infection. Finally, we review advances in avian malaria management in vulnerable bird populations including genetic modification methods suitable for limiting the effects of this disease in wild populations and the use of sterile insect techniques to reduce vector abundance.

Experimental Hut Trials Reveal That CYP6P9a/b P450 Alleles Are Reducing the Efficacy of Pyrethroid-Only Olyset Net against the Malaria Vector Anopheles funestus but PBO-Based Olyset Plus Net Remains Effective

Malaria remains a major public health concern in Africa. Metabolic resistance in major malaria vectors such as An. funestus is jeopardizing the effectiveness of long-lasting insecticidal nets (LLINs) to control malaria. Here, we used experimental hut trials (EHTs) to investigate the impact of cytochrome P450-based resistance on the efficacy of PBO-based net (Olyset Plus) compared to a permethrin-only net (Olyset), revealing a greater loss of efficacy for the latter. EHT performed with progenies of F5 crossing between the An. funestus pyrethroid-resistant strain FUMOZ and the pyrethroid-susceptible strain FANG revealed that PBO-based nets (Olyset Plus) induced a significantly higher mortality rate (99.1%) than pyrethroid-only nets (Olyset) (56.7%) (p < 0.0001). The blood-feeding rate was higher in Olyset compared to Olyset Plus (11.6% vs. 5.6%; p = 0.013). Genotyping the CYP6P9a/b and the intergenic 6.5 kb structural variant (SV) resistance alleles showed that, for both nets, homozygote-resistant mosquitoes have a greater ability to blood-feed than the susceptible mosquitoes. Homozygote-resistant genotypes significantly survived more with Olyset after cone assays (e.g., CYP6P9a OR = 34.6; p < 0.0001) than homozygote-susceptible mosquitoes. A similar but lower correlation was seen with Olyset Plus (OR = 6.4; p < 0.001). Genotyping EHT samples confirmed that CYP6P9a/b and 6.5 kb_SV homozygote-resistant mosquitoes survive and blood-feed significantly better than homozygote-susceptible mosquitoes when exposed to Olyset. Our findings highlight the negative impact of P450-based resistance on pyrethroid-only nets, further supporting that PBO nets, such as Olyset Plus, are a better solution in areas of P450-mediated resistance to pyrethroids.

Overexpression of PxαE14 Contributing to Detoxification of Multiple Insecticides in Plutella xylostella (L.)

The diamondback moth, Plutella xylostella (L.), has evolved with varying degrees of resistance to almost all major classes of insecticides and has become the most resistant pest worldwide. The multiresistance to different types of insecticides has been frequently reported in P. xylostella, but little is known about the mechanism. In this study, a carboxylesterase (CarE) gene, PxαE14, was found significantly overexpressed in a field-evolved multiresistant P. xylostella population and can be dramatically induced by eight of nine tested insecticides. Results of the real-time quantitative polymerase chain reaction (RT-qPCR) showed that PxαE14 was predominantly expressed in the midgut and malpighian tubule of larvae. Knockdown of PxαE14 dramatically increased the susceptibility of the larvae to β-cypermethrin, bifenthrin, chlorpyrifos, fenvalerate, malathion, and phoxim, while overexpression of PxαE14 in Drosophila melanogaster increased the tolerance of the fruit flies to these insecticides obviously. More importantly, gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay showed that the recombinant PxαE14 expressed in Escherichia coli exhibited metabolic activity against the six insecticides. The homology modeling, molecular docking, and molecular dynamics simulation analyses showed that these six insecticides could stably bind to PxαE14. Taken together, these results demonstrate that constitutive and inductive overexpression of PxαE14 contributes to detoxification of multiple insecticides involved in multiresistance in P. xylostella. Our findings provide evidence for understanding the molecular mechanisms underlying the multiresistance in insect pests.

Assessing Insecticide Susceptibility and Resistance Intensity of Anopheles gambiae s.l. Populations From Some Districts of Benin Republic, West Africa

Pyrethroid resistance is widespread in sub-Saharan Africa. The objective of this study was to assess the insecticide resistance intensity in Anopheles gambiae s.l. (Diptera: Culicidae) in four districts of Benin in order to better understand how pyrethroid-only nets are likely to be effective. Thus, adult females of An. gambiae s.l., reared from field-collected larvae were used for assessing resistance intensity to permethrin and deltamethrin. They were tested at 1×, 5×, and 10× the diagnostic dose, using both WHO susceptibility tube testing and CDC bottle bioassays. Identification of molecular species, as well as of L1014F Kdr and Ace-1R mutations was performed using the PCR. The level of expression of biochemical enzymes was also evaluated. Overall, moderate to high resistance intensity to permethrin and deltamethrin was observed, irrespective of the testing method. While the L1014F Kdr frequency was high (>75%), Ace-1R was low (≤6%) in An. gambiaes.s. and Anopheles coluzzii, the two predominant species [52% (95% CI: 44.8-59.1) and 45% (95% CI: 38.0-52.2), respectively]. Anopheles arabiensis was found at very low frequency (3%, 95%CI: 1.1-6.4). For Biochemical analyses, α and β-esterases were over-expressed in all four districts, while mixed-function oxidases (MFOs) were over-expressed in only one. Overall, the two testing methods led to comparable conclusions, though there were a few inconsistencies between them. The moderate-high resistance intensity observed in the study area suggests that dual active-ingredient (AI) long-lasting insecticidal nets (LLINs) may provide better control of insecticide-resistant mosquitoes.

Behavioral responses of pyrethroid resistant and susceptible Anopheles gambiae mosquitoes to insecticide treated bed net

Background

Long-lasting insecticidal nets are an effective tool in reducing malaria transmission. However, with increasing insecticide resistance little is known about how physiologically resistant malaria vectors behave around a human-occupied bed net, despite their importance in malaria transmission. We used the Mbita bednet trap to assess the host-seeking behavior of insecticide-resistant Anopheles gambiae mosquitoes under semi-field conditions. The trap incorporates a mosquito netting panel which acts as a mechanical barrier that prevents host-seeking mosquitoes from reaching the human host baiting the trap.

Methods

Susceptible and pyrethroid-resistant colonies of female Anopheles gambiae mosquitoes aged 3–5 days old were used in this study. The laboratory-bred mosquitoes were color-marked with fluorescent powders and released inside a semi-field environment where a human subject slept inside a bednet trap erected in a traditional African hut. The netting panel inside the trap was either untreated (control) or deltamethrin-impregnated. The mosquitoes were released outside the hut. Only female mosquitoes were used. A window exit trap was installed on the hut to catch mosquitoes exiting the hut. A prokopack aspirator was used to collect indoor and outdoor resting mosquitoes. In addition, clay pots were placed outside the hut to collect outdoor resting mosquitoes. The F1 progeny of wild-caught mosquitoes were also used in these experiments.

Results

The mean number of resistant mosquitoes trapped in the deltamethrin-impregnated bed net trap was higher (mean = 50.21± 3.7) compared to susceptible counterparts (mean + 22.4 ± 1.31) (OR = 1.445; P<0.001). More susceptible mosquitoes were trapped in an untreated (mean = 51.9 ± 3.6) compared to a deltamethrin-treated bed net trap (mean = 22.4 ± 1.3) (OR = 2.65; P<0.001). Resistant mosquitoes were less likely to exit the house when a treated bed net was present compared to the susceptible mosquitoes. The number of susceptible mosquitoes caught resting outdoors (mean + 28.6 ± 2.22) when a treated bed net was hanged was higher than when untreated bednet was present inside the hut (mean = 4.6 ± 0.74). The susceptible females were 2.3 times more likely to stay outdoors away from the treated bed net (OR = 2.25; 95% CI = [1.7–2.9]; P<0.001).

Conclusion

The results show that deltamethrin-treatment of netting panels inside the bednet trap did not alter the host-seeking behavior of insecticide-resistant female An. gambiae mosquitoes. On the contrary, susceptible females exited the hut and remained outdoors when a treated net was used. However, further investigations of the behavior of resistant mosquitoes under natural conditions should be undertaken to confirm these observations and improve the current intervention which are threatened by insecticide resistance and altered vector behavior.

A 6.5kb Intergenic Structural Variation Exacerbates the Fitness Cost of P450-Based Metabolic Resistance in the Major African Malaria Vector Anopheles funestus

Metabolic-based resistance to insecticides limit the control of medically important pests, and it is extremely detrimental in the ongoing struggle to control disease vectors. Elucidating the fitness cost of metabolic resistance in major malaria vectors is vital for successful resistance management. We established the fitness cost of the 6.5kb structural variant (6.5kb-sv) between the duplicated CYP6P9a/b P450s using the hybrid strain generated from the crossing between two An. funestus laboratory strains. Furthermore, we assessed the cumulative impact of this marker with the duplicated P450 genes. We established that individuals that were homozygote for the resistant structural variant (SV) presented reduced fecundity and slow development relative to those that were homozygote for the susceptible SV. Furthermore, we observed that 6.5kb act additively with CYP6P9a and CYP6P9b to exacerbate the reduced fecundity and the increased development time of resistant mosquitoes since double/triple homozygote susceptible (SS/SS/SS) significantly laid more eggs and developed faster than other genotypes. Moreover, a restoration of susceptibility was noted over 10 generations in the insecticide-free environment with an increased proportion of susceptible individuals. This study highlights the negative impact of multiple P450-based resistance on the key physiological traits of malaria vectors. Such high fitness costs suggest that in the absence of selection pressure, the resistant individuals will be outcompeted in the field. Therefore, this should encourage future strategies based on the rotation of insecticides to reduce selection pressure and to slow the spread of pyrethroid resistance.

Knockdown Resistance Mutations in the Voltage-Gated Sodium Channel of Aedes aegypti (Diptera: Culicidae) in Myanmar

Simple Summary

Knockdown resistance (kdr) mutations in the voltage-gated sodium channel (VGSC) of mosquitoes confer resistance to pyrethroid insecticides. Analysis of kdr mutations in Aedes aegypti mosquitoes collected from five different townships in the Mandalay area, Myanmar, revealed high levels of validated kdr mutations in domains II and III of vgsc. Moreover, high frequencies of concurrent kdr mutations were also detected. The results of this study suggest that kdr mutations associated with pyrethroid resistance are widespread in the Ae. aegypti population of the study area. Our results provide a valuable molecular basis to understand the pyrethroid resistance status of the Ae. aegypti population in the area and underscore the need for an effective vector control program in Myanmar.

Abstract

Aedes aegypti is an important mosquito vector transmitting diverse arboviral diseases in Myanmar. Pyrethroid insecticides have been widely used in Myanmar as the key mosquito control measure, but the efforts are constrained by increasing resistance. Knockdown resistance (kdr) mutations in the voltage-gated sodium channel (VGSC) are related to pyrethroid resistance in Ae. aegypti. We analyzed the patterns and distributions of the kdr mutations in Ae. aegypti in the Mandalay area of Myanmar. The segment 6 regions of domains II and III of vgsc were separately amplified from individual Ae. aegypti genomic DNA via polymerase chain reaction. The amplified gene fragments were sequenced. High proportions of three major kdr mutations, including S989P (54.8%), V1016G (73.6%), and F1534C (69.5%), were detected in the vgsc of Ae. aegypti from all studied areas. Other kdr mutations, T1520I and F1534L, were also found. These kdr mutations represent 11 distinct haplotypes of the vgsc population. The S989P/V1016G/F1534C was the most prevalent, followed by S989P/V1016V and V1016G/F1534C. A quadruple mutation, S989P/V1016G/T1520I/F1534C, was also identified. High frequencies of concurrent kdr mutations were observed in vgsc of Myanmar Ae. aegypti, suggesting a high level of pyrethroid resistance in the population. These findings underscore the need for an effective vector control program in Myanmar.

Chirality Effects on Repellent Properties of 4-Alkoxycoumarins Against Asian Tiger Mosquito (Diptera: Culicidae)

The use of semiochemicals as repellents and attractants has been proposed to complement insecticides used for the control of vector mosquito populations. In several studies, the optical purities of the molecules tested have been described as having little or no effect on repellent activity. However, these observations seem difficult to explain because of the chirality effect of molecules on the olfactory system of insects and humans. Thus, the purpose of this study is to assess the effects of chirality on the repellent properties of 4-alcoxycoumarins against Aedes albopictus Skuse, mosquito vector of arboviruses. We report here that the racemic (R/S)-4-sec-butoxycoumarin had the highest repellent effect (Repellent Index = 49.9%) followed by (R) enantiomer (Repellent Index = 24.2%) for the dose of 5 mg/mL. Contrary, no significant repellent activity was recorded for S-(+)-4-sec-butoxycoumarin. This experiment demonstrates the close relationship between the molecules' optical purities and the behavioral response of mosquitoes.

Towards understanding transfluthrin efficacy in a pyrethroid-resistant strain of the malaria vector Anopheles funestus with special reference to cytochrome P450-mediated detoxification

Malaria vector control interventions rely heavily on the application of insecticides against anopheline mosquitoes, in particular the fast-acting pyrethroids that target insect voltage-gated sodium channels (VGSC). Frequent applications of pyrethroids have resulted in resistance development in the major malaria vectors including Anopheles funestus, where resistance is primarily metabolic and driven by the overexpression of microsomal cytochrome P450 monooxygenases (P450s). Here we examined the pattern of cross-resistance of the pyrethroid-resistant An. funestus strain FUMOZ-R towards transfluthrin and multi-halogenated benzyl derivatives, permethrin, cypermethrin and deltamethrin in comparison to the susceptible reference strain FANG. Transfluthrin and two multi-fluorinated derivatives exhibited micromolar potency - comparable to permethrin - to functionally expressed dipteran VGSC in a cell-based cation influx assay. The activity of transfluthrin and its derivatives on VGSC was strongly correlated with their contact efficacy against strain FUMOZ-R, although no such correlation was obtained for the other pyrethroids due to their rapid detoxification by the resistant strain. The low resistance levels for transfluthrin and derivatives in strain FUMOZ-R were only weakly synergized by known P450 inhibitors such as piperonyl butoxide (PBO), triflumizole and 1-aminobenzotriazole (1-ABT). In contrast, deltamethrin toxicity in FUMOZ-R was synergized > 100-fold by all three P450 inhibitors. The biochemical profiling of a range of fluorescent resorufin and coumarin compounds against FANG and FUMOZ-R microsomes identified 7-benzyloxymethoxy-4-trifluoromethylcoumarin (BOMFC) as a highly sensitive probe substrate for P450 activity. BOMFC was used to develop a fluorescence-based high-throughput screening assay to measure the P450 inhibitory action of potential synergists. Azole fungicides prochloraz and triflumizole were identified as extremely potent nanomolar inhibitors of microsomal P450s, strongly synergizing deltamethrin toxicity in An. funestus. Overall, the present study contributed to the understanding of transfluthrin efficacy at the molecular and organismal level and identified azole compounds with potential to synergize pyrethroid efficacy in malaria vectors.

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Transfluthrin and derivatives lack cross-resistance in resistant Anopheles funestus.

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Pyrethroid resistance in An. funestus is strongly synergized by azole fungicides.

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BOMFC is a highly active fluorescent probe substrate for microsomal cytochrome P450 monooxygenases in An. funestus.

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Azole fungicides are nanomolar inhibitors of microsomal cytochrome P450 monooxygenases in An. funestus.

In vivo functional validation of the V402L voltage gated sodium channel mutation in the malaria vector An. gambiae

BACKGROUND

Pyrethroids are the most widely used insecticides for the control of malaria transmitting Anopheles gambiae mosquitoes and rapid increase in resistance to this insecticide class is of major concern. Pyrethroids target the Voltage Gated Sodium Channels (VGSCs), that have a key role in the normal function of the mosquitoes' nervous system. VGSC mutations L995F and L995S have long been associated with pyrethroid resistance and screening for their presence is routine in insecticide resistance management programs. Recently, a VGSC haplotype containing two amino acid substitutions associated with resistance in other species, V402L and I1527T, was identified. These two VGSC mutations are found in tight linkage and are mutually exclusive to the classical L995F/S mutations.

RESULTS

We identify the presence of the V402L-I1527T haplotype in resistant An. coluzzii colonized strains and in field populations from Burkina Faso, at frequencies higher than previously reported; in some cases almost reaching fixation. Functional validation of V402L in insecticide resistance using a CRISPR/Cas9 genome modified line showed that it confers reduced mortality after exposure to all tested pyrethroids and DDT, but at lower levels compared to L995F. In contrast to L995F however, no fitness costs were identified for mosquitoes carrying V402L under laboratory conditions.

CONCLUSION

The V402L substitution confers pyrethroid resistance in An. gambiae in the absence of any other VGSC substitution and/or alternative resistance mechanisms. The lower fitness cost associated with this kdr mutation may provide a selective advantage over the classical kdr in some settings and genotyping at this locus should be added in the list of resistant alleles for routine screening.

Multiple insecticide resistance mechanisms in urban population of Anopheles coluzzii (Diptera: culicidae) from Lagos, South-West Nigeria

Malaria is a major public health challenge in Africa with Nigeria accounting for the highest burden of the disease in the world. Vector control has proved to be a highly effective component of malaria control, however, the development and spread of insecticide resistance in major vectors of malaria have been a major challenge. This study assessed resistance mechanisms in Anopheles coluzzii populations from Kosofe, Lagos mainland and Ojo Local Government Areas in Lagos, Nigeria where An. gambiae s.l is resistant to DDT and Permethrin. WHO susceptibility bioassay test was used in determining resistance status of An. coluzzii to discriminating doses of DDT and Permethrin while synergist assay was used to assess the involvement of monooxygenases in resistance development. Sub-species of An. gambiae s.l (An. gambiae and An. coluzzii) were identified using polymerase chain reaction (PCR) and Restriction Fragment Length Polymorphism (PCR-RFLP) while Allele-Specific Polymerase Chain Reaction (AS-PCR) assay was used to detect knockdown mutation (kdr-West; L1014F). Biochemical assays were used in determining the activities of metabolic enzymes. High DDT resistance was recorded in An. coluzzii populations from the three sites. Mortality rate of mosquitoes exposed confirmed Permethrin resistance in Kosofe (50%) and Lagos mainland (48%) but resistance was suspected in Ojo (96%). All specimens tested were confirmed as An. coluzzii with low kdr frequency; 11.6%, 16.4% and 6.7% in Kosofe, Lagos mainland and Ojo respectively. Pre-exposure to synergist (PBO) before exposure to Permethrin led to increased mortality in all populations. Esterase activity was insignificantly overexpressed in Kosofe (p = 0.849) and Lagos mainland (p = 0.229) populations. In contrast, GST activity was significantly lower in populations from Lagos mainland (63.650 ± 9.861; p = 0.007) and Ojo (91.765 ± 4.959; p = 0.042) than Kisumu susceptible strains (120.250 ± 13.972). Monooxygenase activity was higher in Lagos mainland (2.371 ± 0.261) and Ojo (1.361 ± 0.067) populations, albeit significantly in Lagos mainland (p = 0.007) only. Presence of target-site mutation in all populations, increased mortality with pre-exposure to PBO and elevated monooxygenase in Lagos mainland population were confirmed. Multiple resistance mechanisms in some urban populations of An. coluzzii from Lagos, Nigeria calls for appropriate resistance management strategies.

Influence of seasonal migration on the development of the insecticide resistance of oriental armyworm (Mythimna separata) to λ-cyhalothrin

BACKGROUND

The seasonal long-distance migration of pests amongst different seasonal habitats may encounter different degrees of insecticide selection pressure, but knowledge of the evolution of resistance and the underlying mechanisms remains sparse. Here, we show that the development of resistance of the oriental armyworm Mythimna separata (Walker), a notorious agricultural pest that migrates between northern and northeast China seasonally across the Bohai Gulf, is influenced by seasonal migration.

RESULTS

There are two conspicuous migrations of M. separata, the northeastwards population in the second half of May and the returning population in the second half of August, between northern and northeast China per year, and the abundance values of migrants from April to October are significantly different. The resistance levels of seasonal migratory populations to λ-cyhalothrin vary in different months, and the resistance levels and estimated frequency of the resistance allele of the first northeastward population (second half of May, May.-2) in spring are significantly higher than those of the returning populations in autumn. Moreover, resistance decline was observed in migrating population Jul.-2, with the resistance levels in the F₁ progeny higher than those in their F₂ progeny. Synergism tests indicate that cytochrome P450 monooxygenases (P450s) play a major role in resistance. Enzymatic assays show that P450 activity is significantly correlated with resistance levels in migratory populations, indicating that increased P450 activity is the main mechanism of resistance. The expression levels and correlation analysis of the relative expression of P450s with resistance levels show that three (CYP9A144, CYP9G40, and CYP6B79) out of 23 genes from CYP6 and CYP9 subfamilies are potentially involved in resistance to λ-cyhalothrin.

CONCLUSION

Our results show that the resistance of M. separata to λ-cyhalothrin is unstable and likely to be metabolically driven by enhanced P450 activity mediated by the overexpression of multiple P450 genes. Notably, the development of resistance is probably affected by seasonal migration.

Insecticide Resistance in Aedes aegypti Varies Seasonally and Geographically in Texas/Mexico Border Cities

Insecticide use is the primary method of attempting to reduce or control the spread of mosquito-borne diseases. Insecticide resistance is a major concern as resistance will limit the efficacy of vector-control efforts. The lower Rio Grande Valley region of South Texas has had autochthonous transmission of multiple mosquito-borne diseases including those caused by dengue virus, chikungunya virus, and Zika virus. However, the current status of mosquito resistance to commonly used pesticides in this region is unknown. In this study, we collected field samples from multiple municipalities in South Texas and assessed resistance using the Centers for Disease Control and Prevention bottle bioassay. All populations exhibited characteristics of resistance, and permethrin was the most effective insecticide with an average mortality rate of 44.78%. Deltamethrin and sumethrin had significantly lower mortality rates of 20.31% and 32.16%, respectively, although neither of these insecticides are commonly used for vector-control activities in this region. Depending on which insecticide was used, there was little significance between each of the 7 cities. Seasonal variation in resistance was observed among the collection sites. Both deltamethrin and sumethrin exhibited an increase in susceptibility over the course of 10 months, while permethrin exhibited a decrease in susceptibility. These data highlight the need for further studies to determine if variations in resistance observed are repeated. The data and future findings may be useful in determining the most effective strategies for pesticide use and rotation.

Persistent susceptibility of Aedes aegypti to eugenol

Botanical insecticides are preferred for their environment and user-friendly nature. Eugenol is a plant-based monoterpene having multifarious biocidal activities. To understand whether eugenol would persistently work against Aedes aegypti, we performed larvicidal bioassays on thirty successive generations and determined median lethal concentration (LC50) on each generation. Results showed no apparent differences between LC50 at F0 (63.48 ppm) and F30 (64.50 ppm) indicating no alteration of susceptibility toward eugenol. To analyze, if eugenol has any effect on metabolic detoxification-associated enzymes, we measured esterases (alpha and beta), cytochrome P450, and GST activities from the survived larvae exposed to LC50 concentration from F0–F30. Results revealed a decrease of esterases, GST, and cytochrome P450 activities at the initial 4–8 generations and then a gradual increase as the generations progressed. GST activity remained significantly below the control groups. Synergists (TPP, DEM, and PBO) were applied along with eugenol at F30 and LC50 concentration, and the said enzyme activities were recorded. Results showed a noticeable decrease in LC50 and enzyme activities indicating effective inhibitions of the respective enzymes. Overall, present results inferred that eugenol would effectively work as a larvicide for a longer period in successive generations without initiating rapid resistance and therefore could be advocated for controlling A. aegypti.

Plant-Based Bioinsecticides for Mosquito Control: Impact on Insecticide Resistance and Disease Transmission

The use of synthetic insecticides has been a solution to reduce mosquito-borne disease transmission for decades. Currently, no single intervention is sufficient to reduce the global disease burden caused by mosquitoes. Problems associated with extensive usage of synthetic compounds have increased substantially which makes mosquito-borne disease elimination and prevention more difficult over the years. Thus, it is crucial that much safer and effective mosquito control strategies are developed. Natural compounds from plants have been efficiently used to fight insect pests for a long time. Plant-based bioinsecticides are now considered a much safer and less toxic alternative to synthetic compounds. Here, we discuss candidate plant-based compounds that show larvicidal, adulticidal, and repellent properties. Our discussion also includes their mode of action and potential impact in mosquito disease transmission and circumvention of resistance. This review improves our knowledge on plant-based bioinsecticides and the potential for the development of state-of-the-art mosquito control strategies.

Exposure to a sublethal dose of imidacloprid induces cellular and physiological changes in Periplaneta americana: Involvement of α2 nicotinic acetylcholine subunit in imidacloprid sensitivity

Neonicotinoids are the most important class of insecticides used as pest management tools during several decades. Exposition of insect to sublethal dose of insecticide induces physiological and cellular changes that could contribute to the adaptation of the insects in order to loss their sensitivity to insecticides. The aim of our study is to demonstrate that a subchronic exposure to a sublethal dose of a neonicotinoid imidacloprid is sufficient to induce molecular changes leading to a loss of imidacloprid sensitivity. We report that in the cockroach, Periplaneta americana, subchronic exposure to a sublethal dose of imidacloprid induced weak changes in detoxification enzyme activity and a significant decrease of the nicotinic acetylcholine α2 mRNA. This molecular effect is correlated to a decrease of imidacloprid sensitivity of cockroaches. Using RNA interference, we shown the key role of nicotinic acetylcholine α2 subunit in imidacloprid sensitivity. Thus, quantitative changes in insecticide targets lead to decreased sensitivity to insecticides. This parameter needs to be considered in order to develop sustainable insect resistance management strategies.

Multi-insecticide resistant malaria vectors in the field remain susceptible to malathion, despite the presence of Ace1 point mutations

Insecticide resistance in Anopheles mosquitoes is seriously threatening the success of insecticide-based malaria vector control. Surveillance of insecticide resistance in mosquito populations and identifying the underlying mechanisms enables optimisation of vector control strategies. Here, we investigated the molecular mechanisms of insecticide resistance in three Anopheles coluzzii field populations from southern Côte d’Ivoire, including Agboville, Dabou and Tiassalé. All three populations were resistant to bendiocarb, deltamethrin and DDT, but not or only very weakly resistant to malathion. The absence of malathion resistance is an unexpected result because we found the acetylcholinesterase mutation Ace1-G280S at high frequencies, which would typically confer cross-resistance to carbamates and organophosphates, including malathion. Notably, Tiassalé was the most susceptible population to malathion while being the most resistant one to the pyrethroid deltamethrin. The resistance ratio to deltamethrin between Tiassalé and the laboratory reference colony was 1,800 fold. By sequencing the transcriptome of individual mosquitoes, we found numerous cytochrome P450-dependent monooxygenases – including CYP6M2, CYP6P2, CYP6P3, CYP6P4 and CYP6P5 – overexpressed in all three field populations. This could be an indication for negative cross-resistance caused by overexpression of pyrethroid-detoxifying cytochrome P450s that may activate pro-insecticides, thereby increasing malathion susceptibility. In addition to the P450s, we found several overexpressed carboxylesterases, glutathione S-transferases and other candidates putatively involved in insecticide resistance.

Insecticide-based mosquito control has saved millions of lives from malaria and other vector-borne diseases. However, the emergence and increase of insecticide resistant Anopheles populations seriously threaten to derail malaria control programmes. Surveillance of insecticide resistance and understanding the underlying molecular mechanisms are key for choosing effective vector control strategies. Here, we characterised the degree and mechanisms of resistance in three malaria vector populations from Côte d’Ivoire. Our key finding was that these multi-insecticide resistant malaria vectors largely remained susceptible to malathion, despite the presence of a mutation in the target enzyme of this organophosphate insecticide that would typically confer resistance. Intriguingly, we found overexpression of metabolic P450 enzymes that are known to detoxify insecticides and activate pro-insecticides such as malathion. It is highly probable that, here, we observed P450-mediated negative cross-resistance for the first time in Anopheles field populations. Negative cross-resistance merits further investigation as advantage could be taken of this phenomenon in the fight against multi-resistant malaria vectors.

Nutritional status significantly affects toxicological endpoints in the CDC bottle bioassay

BACKGROUND

The CDC Bottle Bioassay serves as an inexpensive and effective way to screen field-caught mosquitoes against a wide variety of insecticidal active ingredients and commercial formulations, with the goal of detecting resistant individuals. For this study, we assessed how sucrose-water (10% w/v) feeding status impacted the response of Aedes aegypti mosquitoes to select insecticides.

RESULTS

Starvation for 24 or 48 h decreased permethrin and malathion mean survival time by about 40%, with little difference in the two starvation times. Similar findings were also observed in a pyrethroid-resistant Puerto Rico strain challenged with permethrin, but these effects were less pronounced. To test the impact of mosquito weight, we measured weight under different 48-h nutritional conditions and found that sugar-water-fed and sugar-only-fed individuals were approximately the same weight (ANOVA, Bonferroni post-test P value >0.999) and that individuals fed water only were considerably lighter than either sugar-water-fed (ANOVA, Bonferroni post-test P value = 0.034) or sugar-only-fed individuals (ANOVA, Bonferroni post-test P value = 0.027) but equal in weight to starved mosquitoes (ANOVA, Bonferroni post-test P value >0.99). Of the nutritional challenges, water-only-fed mosquitoes were the most insecticide tolerant (ANOVA, Bonferroni post-test P values to all other treatments <0.01).

CONCLUSIONS

The results indicate insect nutritional status is an important experimental variable, particularly the hydration status of mosquitoes shortly before insecticide exposure. Moreover, as significant differences were observed between starved and component-fed (water, sugar, or sugar and water) mosquitoes in a pyrethroid-resistant strain, some caution is appropriate when interpreting resistance/susceptibility diagnoses with this bioassay. © 2021 Society of Chemical Industry. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Effect of salivary CYP4EM1 and CYP4EM2 gene silencing on the life span of Chagas disease vector Rhodnius prolixus (Hemiptera, Reduviidae) exposed to sublethal dose of deltamethrin

Control of Chagas disease in endemic countries is primarily accomplished through insecticide spraying for triatomine vectors. In this context, pyrethroids are the first-choice insecticide, and the evolution of insect resistance to these insecticides may represent an important barrier to triatomine control. In insects, cytochrome P450s are enzymes involved in the metabolism of xenobiotics and endogenous chemicals that are encoded by genes divided into different families. In this work, we evaluated the role of three Rhodnius prolixus CYP4EM subfamily genes during blood meal and after deltamethrin exposure. CYP4 gene members were expressed in different insect organs (integument, salivary glands (SGs), midgut, fat body and malpighian tubules) at distinct transcriptional levels. CYP4EM1 gene was highly expressed in the SG and was clearly modulated after insect blood meal. Injection of CYP4EM1dsRNA promoted significant reduction in mRNA levels of both CYP4EM1 and CYP4EM2 genes and induced deleterious effects in R. prolixus nymphs subsequently exposed to sublethal doses of deltamethrin (3.4 or 3.8 ng/nymph treated). The higher dose reduced the survival over time and increased susceptibility of R. prolixus nymphs to deltamethrin. A better understanding of this mechanism can help in developing of more efficient strategies to reduce Trypanosoma cruzi vector transmission in Americas.

Comprehensive Detoxification Mechanism Assessment of Red Imported Fire Ant (Solenopsis invicta) against Indoxacarb

The red imported fire ant (Solenopsis invicta) is one of the deadliest invasive ant species that threatens the world by disrupting biodiversity, important functions within a natural ecosystem, and community structure. They are responsible for huge economic losses in the infested countries every year. Synthetic insecticides, especially indoxacarb, have been broadly used to control S. invicta for many years. However, the biochemical response of S. invicta to indoxacarb remains largely undiscovered. Here, we used the sublethal doses of indoxacarb on the S. invicta collected from the eight different cities of Southern China. The alteration in the transcriptome profile of S. invicta following sublethal dosages of indoxacarb was characterized using high-throughput RNA-seq technology. We created 2 libraries, with 50.93 million and 47.44 million clean reads for indoxacarb treatment and control, respectively. A total of 2018 unigenes were regulated after insecticide treatment. Results indicated that a total of 158 differentially expressed genes (DEGs) were identified in the indoxacarb-treated group, of which 100 were significantly upregulated and 58 were downregulated, mostly belonging to the detoxification enzymes, such as AChE, CarE, and GSTs. Furthermore, results showed that most of these DEGs were found in several KEGG pathways, including steroid biosynthesis, other drug metabolizing enzymes, glycerolipid metabolism, chemical carcinogenesis, drug-metabolizing cytochrome P450, glutathione metabolism, glycerophospholipid metabolism, glycolysis/gluconeogenesis, and metabolism of xenobiotics. Together, these findings indicated that indoxacarb causes significant alteration in the transcriptome profile and signaling pathways of S. invicta, providing a foundation for further molecular inquiry.